The effective use of exhaust gas aftertreatment systems assumes their control with respect to their reliability in continuous operation. For this, sensors are required using which, even in long term operation, it is possible, for instance, to ascertain exactly the particle concentration currently present in a combustion exhaust gas. In addition, the use of such sensors should enable a loading prognosis of, for instance, a Diesel particulate filter provided in an exhaust gas system, in order to attain a high degree of system safety and thereby allow the use of more cost-effective filter materials. Resistive soot sensors are particularly suitable for this application, and they draw upon the change in resistance of an interdigital electrode structure, based on the adsorption of soot for the detection of the soot.
A sensor for detecting soot in a fluid flow is described in German Patent Application No. DE 10 2004 046 882 A1, which is developed on the basis of a ceramic substrate. It includes two measuring electrodes, set apart from each other, which are developed as interdigital electrodes, that are exposed to the combustion exhaust gas that is to be tested. If soot deposits between the measuring electrodes, this results in a reduction of the insulation resistance of the ceramic material. This is detected, and a soot concentration in the fluid flow is assigned. A heating element of the sensor makes it possible to rid the electrodes and their surroundings of adsorbed soot particles via a thermal process.
The production of such resistive ceramic particle sensors first takes place on a common ceramic substrate. Finally, a separation of the corresponding sensor elements takes place. Conditioned upon the production tolerances, for instance, when the measuring electrodes of the sensor element are applied using screen printing, they are applied only in a limited area of the sensor surface, so as to avoid that, when the sensor elements are separated, the printed circuit traces of the electrodes are severed and the sensor becomes inoperable. Based on these tolerances, and depending on the positioning within the tolerance range, the electrodes are situated more or less centered on the sensor element. It has turned out, however, that the sensor functionality of a sensor element shows a clear dependence on the positioning of the electrode on the sensor element. Therefore, the production tolerances lead to a specimen variance in the resulting sensor elements, when the sensor elements are separated.